Power utilities generate electrical power at remote plants and deliver electricity to residential, business or industrial customers via transmission networks and distribution grids. The power utilities may transmit large quantities of electric power over long distance transmission networks from power generating plants to regional substations, which then supply the power to local customers using the distribution grids.
The transmission networks and/or distribution grids may include overhead power transmission lines suspended by towers or poles. The transmission lines may, for example, be bare wire conductors made of aluminum. Instead of aluminum, copper wires may be used in medium-voltage distribution and low-voltage connections to customer premises.
Power loss in transmission lines (in particular, in long distance transmission lines) is a significant component of the cost of electricity. This power loss is a decreasing function of transmission voltage. Therefore, power is typically first transmitted as high voltage transmissions from the remote power plants to geographically diverse substations. The most common transmission voltages in use are 765, 500, 400, 220 kV, etc. Transmission voltages higher than 800 kV are also in use. From the substations, the received power is sent using cables or “feeders” to local transformers that further reduce the voltage. Voltages below 69 kV are termed subtransmission or distribution voltages. The outputs of the transformers are connected to a local low voltage power distribution grid that can be tapped directly by the customers.
Any electric power transmission and distribution system (“delivery system”), which includes different complex interacting elements, in operation is susceptible to disturbances, surges and faults. The faults may, for example, include open circuit faults, short circuit faults, earth leakage faults and insulation breakdown. The faults may be produced as a result of, for example, lightning strikes, mechanical loading by ice and/or wind, operation of certain electrical equipment, electromagnetic surges, static electricity, and/or induced voltages. A fault often results in overvoltage transients, travelling wave pulses and uncontrolled release of energy (e.g., arcing), which can cause further damage to the system and attached loads. Accordingly, electric power delivery systems are often provided with surge protectors/shunts (e.g., a crowbar circuit) to divert energy to ground or neutral. Further, the electric power delivery equipment may be then de-energized to allow for fault clearing, recovery or repair.
The surge protectors may include circuitry that is responsive to a rate of change of a current or voltage to prevent a rise above a predetermined value of the current or voltage. In power transmission systems, surge protector circuits may allow the voltage on a transmission line conductor to rise very rapidly when a lightning strike or other surge occurs on the line, until the breakdown voltage of the gas tube, triac, or other crowbar device goes conductive, and the impedance from the conductor to ground or to some other reference potential reduces very rapidly.
The term “switchgear” is commonly used, in the context of electric power delivery systems, to refer to the combination of electrical disconnects, fuses, relays, and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.
Consideration is now being given to solutions for interrupting fault currents and de-energizing equipment in high voltage electrical power delivery systems.